The present invention relates in general to radar and in particular to a new and useful radar system which can be used primarily as ground tracking radar, and which produces a pulse compression signal having asymmetrical sidelobes for reducing distance errors which are caused by symmetrical sidelobes.
Pulse compression methods have already been known in the field or radar, for some time. They are comprhensively described in the publication Modern Radar, John Wiley & Sons, 1967, for example. Pulse compression makes it possible to obtain a higher resolution in distance than would correspond to the length of the transmitted pulse. Developments in this area show a preference for digital techniques, with the transmitted pulse being sub-divided into a plurality of subpulses which are coded in a digital sequence such as a pseudo-random sequence. In a matched-filter type of pulse compression in a radar receiver, an echo pulse reflected by a target is substantially compressed to a pulse which is very short relative to the transmitted and the echo pulse. The degree of compression generally grows better with the number of subpulses. A large number of subpulses, however, is equivalent to a long duration of the transmitted pulse, which is contrary to the use of this technique for measuring very short distances, such as while determining the flying height of aircraft with ground tracking radar.
Analog pulse compression methods known in the radar field and operating with a frequency-modulated transmitted pulse and a compression filter accomplishing a frequency-dependent time lag and weighting, produce relative high time sidelobes associated with the main bode of the compressed target echo signal. Sometimes, these high sidelobes are evaluated instead of the main lobe, which leads to considerable distance measuring errors and, in ground tracking radars, to errors in altitude.